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Shhh, the Ants Are Talking

If you want to survive as an ant, you'd better get ready to make some noise. A new study shows that even ant pupae—a stage between larvae and adult—can communicate via sound, and that this communication can be crucial to their survival.

"What's very cool about this paper is that researchers have shown for the first time that pupae do, in fact, make some sort of a sound," says Phil DeVries, an entomologist at the University of New Orleans in Louisiana who was not involved in the study. "This was a very clever piece of natural history and science."

Scientists have known for decades that ants use a variety of small chemicals known as pheromones to communicate. Perhaps the most classic example is the trail of pheromones the insects place as they walk. Those behind them follow this trail, leading to long lines of ants marching one by one. However, the insects also use pheromones to identify which nest an ant is from and its social status in that nest. Because this chemical communication is so prevalent and complex, researchers long believed that this was the primary way ants shared information.

However, several years ago, researchers began to notice that adults in some ant genuses, such as Myrmica, which contains more than 200 diverse species found across Europe and Asia, made noise. These types of ants have a specialized spike along their abdomen that they stroke with one of their hind legs, similar to dragging the teeth of a comb along the edge of a table. Preliminary studies seemed to indicate that this noise served primarily as an emergency beacon, allowing the ants to shout for help when being threatened by a predator.

Larvae and young pupae have soft outer skeletons, which means their specialized spikes haven't yet formed and they can't make noise. However, as the pupae mature, their covering hardens into a tough exoskeleton like that found in adult ants. These older pupae do have fully functional spikes but were generally thought to be silent.

Talking Ants

Audio adult. M. scabrinodis worker ants create a much more complex sound using the same method.

Credit: Luca Casacci et al., Current Biology

Karsten Schönrogge, an entomologist at the Centre for Ecology & Hydrology, Wallingford, in the United Kingdom, thought it odd that mature pupae would have the capability to produce sound but remain silent. So he and his colleagues listened in to a group of Myrmica scabrinodis ants. These 4- to 5-millimeter-long, reddish-brown ants are commonly found in northern Europe, in low-lying areas like peat bogs.

Using an extra-sensitive microphone that would pick up on the faint acoustic signals, the researchers measured the sounds produced by 10 differentM. scabrinodis larvae, six immature pupae, and six mature pupae. Whereas the larvae and immature pupae were completely silent, the mature pupae produced brief pulses of sound (see audio files), the team reports online today in Current Biology.

Further analysis of this noise showed that it was a simplified version of the more complex adult sound. It was as if the mature pupae were saying, "Help!" while the adults were saying "Hey, I'm over here! Please come help! It's your friend!"

To test the function of these noises in the mature pupae, the researchers first played back the sounds made by either the mature pupae or adult M. scabrinodis. Adult worker ants responded the same way to both recordings, such as walking over to the speaker, rubbing their antennae against it, and guarding it. They didn't show these responses when Schönrogge and colleagues played white noise. These behaviors, which represent a worker ant's attempts to protect its nestmates, indicate that acoustic communication served to bring assistance in both mature pupae and adult ants.

To see how the ants used this acoustic communication, the team removed the abdominal spike from some of the mature pupae in a nest. The researchers then disturbed the nest, spilling out larvae, pupae, and adult workers into an experimental arena. Normally, the adult ants rescue their nestmates in a specific order: mature pupae, immature pupae, and, finally, the larvae. In the experiments by Schönrogge and colleagues, the adult workers indeed rescued the unmuted mature pupae first. However, the adult ants completely ignored the muted ants. It was as if the mute mature pupae simply didn't exist.

"The sounds they make rescue them by signaling their social status," Schönrogge says. "There is complex information in these signals," that combine with chemical signals to provide an array of information about the individual. Researchers have yet to decode everything the ants are communicating by sound and how the ants interpret these signals. Acoustic communication may be especially important in mature pupae because they don't yet produce the full array of adult pheromones, but they also don't smell and behave like larvae, either.

DeVries cautions that the discovery doesn't mean that chemical communication in ants is less important. "Ants live in these enormously sophisticated societies," he says. "Acoustic signaling adds another gorgeous piece to what we know about how insect societies communicate."